PROJECT SUMMARY Hypospadias is a birth defect wherein the urethral opening is displaced from the distal end of the penis and instead terminates on the ventral side of the penis, or in severe cases, on the scrotum or perineum. Hypospadias is one of the most common birth defects, affecting approximately 1 in 125 live male births. Surgery is currently the only treatment, which comes with a risk of complications and recurrence. Recent work has demonstrated that both genetic factors and environmental factors such as embryonic exposure to endocrine disrupting chemicals can increase the rate of hypospadias. Despite these studies, we still do not have a good grasp on what aspects of genitalia morphogenesis become disrupted and result in hypospadias. The embryonic genital tubercle (GT) gives rise to either the penis or clitoris, and contains tissues from all three germ layers. The endodermal component of the GT primarily gives rise to the urethra, mesodermal cells contribute to supportive structures within the external genitalia, and the ectoderm gives rise to the overlying skin. During GT development, the endoderm and ectoderm are connected at an endoderm/ectoderm junction (EEJ) on the ventral side of the GT, forming one of very few sites in the body where such a junction occurs. This junction is remodeled away over the course of penis development and urethral internalization, leading to distal positioning of the urethral meatus; the EEJ remains intact during and after clitoral development, resulting in the ventral position of the urethral opening between the clitoris and vagina. This leads to the hypothesis that EEJ remodeling is necessary for urethral internalization in the penis, and disruptions to the EEJ cause hypospadias. To test this hypothesis, this proposal uses mouse mutants in the Fgf signaling pathway as a model to test how disruptions of the EEJ result in GT malformations: mice with Fgfr2 mutations display endodermal and ectodermal defects in the GT and subsequently develop severe hypospadias. Aim 1 of this proposal will test how Fgfr2 regulates cell movements at the EEJ during urethral internalization; this aim makes use of a novel GT organ culture system coupled with four-dimensional live imaging techniques to directly visualize morphogenesis of the mouse EEJ. Aim 2 will examine the biomechanical mechanisms underlying urethral internalization and test the role of extracellular matrix adhesion in contributing to EEJ development. The findings from this proposal will generate an atlas of the cell and tissue movements which occur during urethral formation in the mouse, a resource which is not available for most mammalian organ systems. The research proposed here will not only shine a light onto the morphogenetic mechanisms underlying normal external genitalia development, but will also provide insight into how hypospadias arise and will help shape potential preventative measures for this common congenital defect.